Machine Terminals Research Articles

Wind turbine driven self-excited induction generator

Owing to increased emphasis on renewable resources, the development of suitable isolated power generators driven by energy sources such as wind, small hydro-electric, biogas, etc. have recently assumed greater significance. A capacitor self-excited induction generator has emerged as a suitable candidate of isolated power sources. The utilization of a three-phase squirrel cage induction motor as an autonomous self-excited induction generator is reviewed. Variations of electrical power due to changes in wind speed should be as small as possible. This is obtained by using an induction generator driven by a wind turbine. The system incorporates the use of turbine blade angle pitch control to control wind turbine speeds and shaft torque. A variable pitch turbine operates efficiently over a wider range of wind speeds. Self-excitation with capacitors at the stator terminals of the induction machine is well demonstrated experimentally on a d.c. motor-induction generator set. The parameters and the excitation requirement of the induction machine run in self-excited induction generator mode are determined. The effects of variations in prime mover speed, terminal capacitance and load power factor on the machine terminal voltage are studied.

Analysis of series-connected wound-rotor self-excited induction generator

The paper presents theoretical and experimental investigations for steady-state performance of a three-phase wound-rotor series-connected self-excited induction generator (SCSEIG). This type of generator can be realised by series connection of stator and rotor windings of a slip-ring type induction machine. Self excitation may occur when a suitable capacitor bank is connected across the machine terminals. The generator produces an output voltage with a frequency of half the rotor electrical angular frequency and independent of the load conditions provided that the prime mover speed is kept constant. The well known generalised machine theory has been employed with a proper transformation, resulting in a hypothetical equivalent saliency, which produces periodically time-varying inductances needed for such generators (via the mutual coupling between rotor and stator windings). Analysis is achieved by considering saturation effects on both axes (d and q) and theoretical results were found to agree well with corresponding experimental values

Application of Floquet's theory to the analysis of series-connected wound-rotor self-excited synchronous generator

In this paper, Floquet's theory for solving differential equations with periodically varying coefficients has been utilized in evaluating the steady state performance of a three phase wound rotor series-connected self-excited synchronous generator SCSESG. This type of generator is practically realized by the series connection of stator and rotor windings of a conventional wound-rotor induction machine. Self excitation may occur when a suitable capacitor bank is connected across the machine terminals. The analysis gives the same results that are obtained when the d-q transformation model is utilized. Application of Floquet's theory has the advantage of reducing the mathematical manipulation needed. The results are checked experimentally. Saturation effects on each axis inductance as well as iron losses show satisfactory agreement. The generator acts as a hypothetical salient pole machine operating at half the rotor electrical angular frequency and is independent of load conditions provided that the prime mover speed is kept constant.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Voltage regulator for an isolated self-excited cage induction generator

A voltage regulator for a three-phase self-excited cage induction generator (SEIG) is developed for maintaining its terminal voltage constant irrespective of the nature and amount of load. The proposed regulating scheme consists of a fixed-value capacitor bank and saturable core reactors with appropriate feedbacks. This simple static exciter at the induction machine terminals is capable of meeting its reactive power demand and regulating the terminal voltage. The developed regulator is used for a 5 hp, three-phase squirrel cage induction generator and provides inherent voltage regulation at resistive, inductive and motor loads. Extensive tests are carried out to study the steady-state and dynamic performance of the system. The relevant recorded voltage and current waveforms and other test results are given and discussed in detail. The proposed regulator is also compared with a thyristor controlled reactor type of regulator for an SEIG and is found to be superior to the latter in several aspects.

Analysis Of Heating And Losses In Generator Field Discharge Resistors And Generator Rotors Following Severe Disturbances On The Electrical Supply System

ABSTRACT The paper examines heating and losses in generator field discharge resistors and generator rotors following sustained stator-terminal and HV busbar L-L short circuits at full-load and no-load. Losses in generator rotors following L-L-L short-circuits on a weak line which is connected to the HV generator transformer busbar with clearance at fault current zeros where the generator either remains in synchronism or falls from synchronism, and following worst case malsynchronisation are also reviewed. Comparisons are made with negative sequence losses in solid generator rotors following these disturbances given by I2 2t computed from detailed analysis for both loaded and unloaded machines. The analysis uses a phase-variable model of a synchronous-generator with detailed and reduced damper representations to compute stator and rotor current following a severe electrical disturbance at either the machine terminals or at the HV unit transformer busbar. Simulations at full-load and no-load for a variety of assumptions and approximations, with connection of the field discharge resistor on tripping the main generator breaker are performed. The investigations are undertaken using data for a 660MW, 3000 rpm and a 500MW, 3600 rpm Machine.

LOAD COMMUTATED INVERTER FED VARIABLE SPEED RELUCTANCE MOTOR DRIVE

ABSTRACT The naturally commutated d.c. link inverter is employed for feeding the active power to the reluctance motor. The lagging reactive power requirements of the motor and inverter fulfilled by connecting the capacitor bank at machine terminals. A wide range of speed control is obtained by varying the d.c. link voltage and value of the terminal capacitors.

Comparison of losses and heating in generator rotors following severe supply system disturbances

Losses and heating in rotors of large synchronous generators are examined following sustained stator-terminal and HV busbar line-to-line short-circuits at full load and no load, three-phase (L-L-L) short-circuits on a weak line connected to the HV generator transformer busbar with clearance at fault current zeros where the generator either remains in synchronism or falls from synchronism, and worst-case malsynchronization. Comparisons are made with negative sequence losses in solid generator rotors following these disturbances. The analysis uses a phase-variable model of a synchronous generator with detailed and reduced damper representations to compute stator and rotor current following a severe electrical disturbance at either the machine terminals or the HV unit transformer busbar. Simulations at full load and no load for a variety of assumptions and approximations, with connection of a field discharge resistor on tripping the main generator breaker, are performed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Coupling of discharge currents between conductors of electrical machines owing to laminated steel core

Over the past few years, a number of techniques have evolved for detecting discharges in the insulation systems of electrical machines by measuring the discharge currents flowing at the machine terminals. The propagation of these currents through the winding, from the discharge site to the terminals, is influenced by the capacitive and inductive coupling between the conductors of the winding. The paper is concerned solely with the inductive component of that coupling, through the laminated steel stator core of the machine. It describes how this coupling makes a significant contribution to the equivalent circuit for the propagation of discharge currents containing frequencies from 20 kHz to 20 MHz.

Effect of Surge Capacitor Lead Length on Protection of Motors from Steep Switching Surges

The results reported in this paper suggest that capacitor protection of large AC motors may not be adequate to protect against motor starting switching surges when the length of the leads connecting the capacitor to the machine terminals and to ground are substantial. Accordingly, it is shown that the magnitude and shape of the switching surges appearing in the machine windings depend upon the length of the capacitor connecting leads. It is shown that steep switching surges with high rates of rise can be impressed on motor winding insulation when the surge capacitor leads to machine terminals and to ground are long.

Transient stability of an a.c. generator with governor action by use of the phase space method and an analog computer

The object of the paper is the study of the transient stability of a synchronous machine connected to an infinite bus by utilizing the phase space method, assuming constant voltage behind transient reactance, constant damping and variable mechanical power input. The swing equation including governor action, after a 3 phase fault at the machine terminals is a third-order nonlinear differential equation.

Induction Generator Applications for Petroleum and Chemical Plants

A 13 900-hp 1200-r/min induction machine has been applied as a motor/generator in a compressor-expander string associated with a fluid catalytic cracking (FCC) process. Power factor correction capacitors are connected at the terminals of the induction machine. The specific principles of application, as they apply to generator applications, of induction machines are reviewed. Mechanical design, overspeed, excitation, electrical and mechanical transients, and machine control are studied in detail.

Damping Subsynchronous Resonance Using Reactive Power Control

The IEEE benchmark system is used to demonstrate a method of damping subsynchronous resonance using a set of phase controlled reactors connected to the machine terminals. It is possible to prevent one mode of instability by modulating the reactive power consumed by the reactors in proportion to the change in generator speed. However, it is shown that this previously Suggested method of control is ineffective in damping all possible modes of instability of the benchmark system due to subsynchronous resonance. Methods of control are established which can be used to damp all critical modes of instability using a set of phase controlled reactors

Improvement of the Dynamic Stability Performance of Multimachine Power Systems by the Use of Optimized Static Var Compensators

This paper presents the dynamic performance analysis of a multimachine power system operating with thyristor controlled static VAR compensators at the machine terminals. The synchronous machines of the multimachine power system are modelled using d-q axes quantities and the thyristor controlled static, VAR compensator as a inertialess variable voltage source with a fixed reactance. A biasing capacitor is connected in parallel with the controlled reactor to extend the operating range to both inductive and capacitive range, of the reactive power. The magnitude of' the voltage source however, depends on the firing angle of the thyristors forming the reactor compensator arms. In order to make the thyristor controlled static compensator effective over a wide range of system operating conditions, a variable structure stabilizing control loop with several gains and time constants is used along with auxillary feedback signals proportional to speed, terminal voltage and reactive power deviations.An eigenvalue search technique is used for the conbined state space model of the multimachine power system and static VAR compensator to optimize the structure of the stabilizing control loop for the best system damping. A two machine power system is considered for digital simulation to highlight the static compensator perfermance for improving the performance of the system for both small and large signal disturbances. From the simulation it is found that the system dynamic performance can be improved considerably during small disturbance conditions and faults and load rejection at the machine terminals using a static compensator. However when the rating of the compensator is limited to a smaller value( for economic considerations) it operates primarily in the bane-bang control node and therefore in place of a continuous controller, a switching controller With fixed inductor and capacitor component is feasible.

Distribution of switching surges in the line-end coils of cable-connected motors

When specifying the winding insulation of electrical machines it is necessary to know the electrical stresses to which the winding will be subjected. In the case of interturn insulation the most severe stresses are caused by steep-fronted voltage waves produced by circuit-breaker closure. To some extent such steepfronted waves are reduced in severity by the time they reach the machine terminals by distortion and attenuation in the cable connecting the machine to the circuit breaker. In the studies described the interturn voltages in the line-end coils of cable-connected machines are evaluated. The equivalent circuits for the machine winding, the cable, and the energising source are obtained and compounded into an equivalent circuit for the complete system. The interturn voltages in the line-end coils of the machine are obtained from solutions of this equivalent circuit. Studies are made with a number of different cable lengths and types, and for a number of energisation source representations. Single-pole and 3-pole simultaneous closure of the circuit breaker are considered. The use of series inductors at the circuit breaker, or shunt capacitors at the motor terminals, for reducing the severity of the interturn voltages is also investigated. The results obtained indicate that the type of cable insulation and the length of cable are both significant in influencing the magnitude of interturn voltages. Energisation source impedance has a large effect. Comparison of seriesinductance and shunt-capacitance methods of surge protection reveals that both methods are effective, but that both produce other unwanted effects.

EQUIVALENT CIRCUITS, TIME CONSTANTS, AND REACTANCES OF DUAL-EXCITED SYNCHRONOUS MACHINES

ABSTRACT The generalized transient equivalent circuits of dual-excited synchronous machines, in which the two field windings are not necessarily located on the rotor axes and may have different number of turns as well as different inclination angles to the physical-axis of the rotor, are developed. It is found that, in this case, the machine circuits on both the d- and q-axes are interacting and are no longer independent of each other. The equivalent circuits of machines with special construction, namely: conventional, d-q and d.w.r. synchronous machines can be derived as special cases of these generalized equivalent circuits. Expressions for the currents following the sudden application of a symmetrical 3-phase short-circuit at the terminals of a dual-excited synchronous machine are derived. The different approaches for finding out formulae for the different machine time constants and the constraints imposed on each are presented.

Fast generator protection against internal asymmetrical faults

An on-line digital computer technique for the protection of a generator against internal asymmetrical faults is described in this paper. The proposed technique relies on the detection of a second harmonic component in the field current at the onset of a fault in the armature winding. Discrimination against external faults is achieved by monitoring the direction of the negative sequence power flow at the machine terminals. Tests have been conducted on a laboratory synchronous generator for various types of internal and external faults. Results show that the total time for internal fault detection and tripping is well within half a cycle of the 60 Hz signal. A simple extension to negative sequence relaying and loss of excitation relaying is proposed.

Shaft torques during out-of-phase synchronization

As a consequence of shaft failures in two units in a Consumers Power gas turbine installation, a computer study was Performed to determine the torques In the turbine-generator shaft system during out-of-phase synchronization. Prior to this study, the possible occurrence of excessive torques in the turbine-generator shaft system without damage to the stator windings due to large currents was questioned. If the gas turbine-generator unit is designed to sustain the stator currents and the instantaneous shaft torques resulting from a three phase short circuit at the machine terminals, then the results of this investigation revealed that during out-of-phase synchronization the design torques may be exceeded by 2 to 3 times without exceeding the amplitude of the short circuit stator currents. From this it seems reasonable to infer that shaft failure could occur during out-of-phase synchronization without damage to stator windings due to excessive currents.

An Adaptive Voltage Regulator for Coordinated Control of Machine Excitation Systems

The Coordinated control of individual machine excitation in a multimachine system may offer improved system performance during disturbances. Inherent in this type of control is the need to alter the characteristics of individual excitation systems in order to satisfy some overall coordination performance index. An adaptation strategy is needed to achieve the most suitable regulator operation for steady state conditions and for varying degrees of transient disturbance.A strategy is suggested in this paper for adapting an excitation system toward prevailing conditions as seen at the machine terminals. The strategy utilizes the injection of a stabilizing signal. The field voltage forcing achieved by the introduction of the signal is determined by the continuous measurement of a readily available machine state rather than by a discontinuous bang/bang strategy. The characteristics of the feedback loop can be determined by the severity of the fault at the machine terminals.

Analytical method for estimating electromagnetic damping in a multistator stepping motor

An analytical method is developed for predicting the electromagnetic damping of the single-step response of a multistator, variable-reluctance stepping motor operating with multiphase excitation. The analysis, which is based on the essentially sinusoidal variation of the time dependent quantities, takes into account the inherent magnetic nonlinearity. The prediction is made from relatively straightforward static tests at the machine terminals. The analytical results are compared with those obtained from experiment, and from numerical solution of the nonlinear differential equations, for a 5-phase multistator motor.

Method for predicting the dynamic response of a variable-reluctance stepping motor

A method is described for predicting the index response of a reluctance-type stepping motor from the results of static tests at the machine terminals. The method involves solving the system equations by numerical integration once an equivalent circuit representing one phase of the motor has been obtained. To obtain this circuit requires (a) representation of the flux linkages of one phase as a function of current and angular position of the rotor by using a polynomial in the two variables, and (b) representation of eddy currents in the core by suitably chosen resistive circuits in parallel with sections of the coil. A comparision of experimental and predicted responses for a commercially manufactured stepping motor is presented.